This invention relates to a photoacoustic signal detecting method and apparatus utilizing a photoacoustic effect for the detection of surface and internal information of a sample and relates also to a method for detecting an internal defect of a semiconductor device.
The photoacoustic effect described above was discovered by Tyndall, Bell, Rontgen, et al. in 1881 and represents a phenomenon which will be described with reference to FIG. 1. That is, when, as shown in FIG. 1, an intensity-modulated laser beam (an intermittent laser beam) focused by a lens 5 irradiates a sample 7, heat is generated in a light absorption region Vop 21 inside the sample 7 and periodically diffuses through a thermal diffusion region Vth 23 determined by a thermal diffusion length .mu..sub.s 22 thereby inducing a thermal distortion wave, and this thermal distortion wave acts to generate a surface acoustic wave (an ultrasonic wave). Therefore, when this ultrasonic wave, that is, a photoacoustic signal, is detected by a microphone (an acousto-electrical transducer), a piezoelectric transducer element or an interferometer, and a signal component synchronous with the modulation frequency modulating the incident laser beam is then detected, surface and internal information of the sample can be detected. The above manner of detection of a photoacoustic signal is discussed in, for example, "HIHAKAI KENSA (Nondestructive Testing)" Vol. 36, No. 10, pp. 730-736 (Oct., 1987) and "IEEE; 1986 ULTRASONICS SYMPOSIUM", pp. 515-526, (1986). According to these publications, a photoacoustic signal generated as a result of irradiation of a sample with an intensity-modulated laser beam is detected by, for example, an interferometer so as to extract a frequency component synchronous with the modulation frequency modulating the incident laser beam. This extracted frequency component has surface or internal information of the sample corresponding to the modulation frequency. By changing the modulation frequency, the thermal diffusion length .mu..sub.s 21 shown in FIG. 1 can be changed, so that information in the direction of the depth of the sample can be obtained. Therefore, when a crack or any other defect is present inside the thermal diffusion region Vth 23 shown in FIG. 1, a signal level change appears in the extracted frequency component in the range of the interferometry intensity signal, so that the presence of the defect can be detected. However, although the prior art method described above is a very effective means capable of detecting the photoacoustic signal in a noncontact and nondestructive manner, the prior art method has had that difficulty in the case of detection of internal information of a sample having a microstructure of the order of a micron (1 .mu.m) or less (submicron).